There is much information demonstrating the use of semiconducting metal oxides for gas sensors. The reaction mechanism begins with the absorption of ambient oxygen onto the metal oxide surface from the surrounding atmosphere. The adsorbed electro-negative oxygen molecules withdraw electron density from the surface of the metal oxide. This equilibrium shift in the electron density creates a depletion layer which lowers the baseline conductivity of the material relative to vacuum. Referring to FIG. 1, combined with surface structure, this depletion layer forms a potential barrier against electron flow. When the material is heated to several hundred degrees Celsius, the oxygen takes on a reactive form, O−ads. This adsorbed oxygen ion species can now react with the analyte gas so that a charge transfer reaction takes place. In one example, carbon monoxide reacts with the oxygen to form carbon dioxide. The generation of an electron on the semiconducting metal oxide surface by this reaction produces an increase in current. The amount of current is proportional to the concentration of CO that reacts with the adsorbed oxygen species on the metal oxide surface.
A typical metal oxide material for this reaction is tin (IV) oxide. To establish the conditions for the above reaction to occur, the surface temperature of SnO2 is higher than 280° C. The amount of power required to heat this material to several hundred degrees is very large, usually exceeding several watts. This large power consumption greatly reduces the ability to power a sensor using a battery. While not impossible, the battery lifetime for operation is very short.
An example of this power requirement is the Taguchi gas sensor (“TGS”) manufactured by Figaro Engineering Inc. (“Figaro”), which requires periodic heating to achieve high accuracy. The TGS currently pulses its heater once every second in its electronic set, with an average power consumption of 14 mW (milliwatts). This high power consumption is prohibitive to a battery-powered application.